Exhaust emission control apparatus for internal combustion engine

ABSTRACT

An exhaust emission control apparatus that is used with an internal combustion engine to occlude and reduce NOx without blocking the exhaust gas purification function of a catalyst. An NOx occlusion reduction type catalyst and ozone supply device are provided in an exhaust path of the internal combustion engine. The NOx occlusion reduction type catalyst includes a cell into which an exhaust gas flows. An inner surface of the cell is coated with a NOx retention layer and a catalyst layer so that the NOx retention layer and the catalyst layer are provided in the order named from the inner surface of the cell.

TECHNICAL FIELD

The present invention relates to an exhaust emission control apparatus for an internal combustion engine.

BACKGROUND ART

It is known that a conventional exhaust emission control apparatus disclosed, for instance, in JP-A-2002-89246 includes an NOx occlusion reduction type catalyst. When the conventional exhaust emission control apparatus is used, an exhaust path of an internal combustion engine is provided with a catalyst and with a substance capable of occluding NOx (hereinafter also referred to as the “NOx retention substance”). Such a configuration is formed so that NOx in an exhaust gas is occluded by the NOx occlusion reduction type catalyst in a lean atmosphere, and that the occluded NOx is released, reduced, and decomposed in a rich atmosphere.

To ensure that the above reaction smoothly takes place, it is preferred that the catalyst reach its activation temperature and fully exercise its activation function. When an internal combustion engine starts up, however, the catalyst temperature is low. Thus, the conventional exhaust emission control apparatus addresses the above problem by adding ozone (O₃) to the exhaust gas at internal combustion engine startup. Adding ozone to the exhaust gas oxidizes NOx in the exhaust gas to accelerate an NOx occlusion reaction. Consequently, even when the catalyst is not fully active at the time, for instance, of internal combustion engine startup, the use of the above-described conventional technology makes it possible to accelerate NOx occlusion and purify the exhaust gas.

-   Patent Document 1: JP-A-2002-89246 -   Patent Document 2: JP-A-1993-192535 -   Patent Document 3: JP-A (PCT) No. 538295/2005 -   Patent Document 4: JP-A-1994-185343 -   Patent Document 5: JP-A-1998-169434 -   Patent Document 6: Japanese Patent No. 3551346

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

Meanwhile, the above-described NOx occlusion reduction type catalyst is formed so that a layer containing a catalyst and an NOx retention substance is coated onto a base material (which may also be referred to as a support). This type of NOx occlusion reduction type catalyst tends to have a smaller exhaust gas purification capacity (a smaller capacity of purifying NOx, HC, and CO) than a conventional three-way catalyst without an NOx retention substance. It means that the exhaust gas purification function of the above-described NOx occlusion reduction type catalyst is blocked.

The present invention has been made to solve the above problem. An object of the present invention is to provide an exhaust emission control apparatus that is used with an internal combustion engine to occlude and reduce NOx without blocking the exhaust gas purification function of a catalyst.

Means for Solving the Problem

To achieve the above-mentioned purpose, the first aspect of the present invention is an exhaust emission control apparatus for an internal combustion engine, comprising:

an NOx occlusion reduction type catalyst which is positioned in an exhaust path of the internal combustion engine; and

ozone supply means which supplies ozone so that the ozone mixes with an exhaust gas flowing into the NOx occlusion reduction type catalyst;

wherein the NOx occlusion reduction type catalyst includes a cell in which the exhaust gas flows, an inner surface of the cell being coated with a first layer and a second layer, the first layer and the second layer being provided in the order named from the inner surface of the cell, the first layer containing an NOx retention substance, the second layer including a noble metal and permitting the passage of NOx, the amount of the NOx retention substance in the second layer is smaller than that in the first layer.

The second aspect of the present invention is the exhaust emission control apparatus according to the first aspect, wherein the amount of the NOx retention substance contained in the second layer is substantially zero.

The third aspect of the present invention is the exhaust emission control apparatus according to the first or the second aspect, further comprising:

ozone supply amount adjustment means for adjusting an ozone supply amount so that the mole ratio of ozone to nitrogen monoxide (NO) in a gas mixture flowing into the NOx occlusion reduction type catalyst is greater than 1.

The fourth aspect of the present invention is the exhaust emission control apparatus according to the third aspect, wherein the ozone supply amount adjustment means adjusts the ozone supply amount so that the mole ratio of ozone (O₃) to nitrogen monoxide (NO) in the gas mixture flowing into the NOx occlusion reduction type catalyst is not smaller than 2.

Advantages of the Invention

According to the first aspect of the present invention, an NOx retention layer and a catalyst layer are independently formed. Therefore, the catalyst can properly exercise its exhaust gas purification function. It is thought that the NOx retention substance is a catalyst poison for a noble metal element and is a factor of decreasing the exhaust gas purification capacity of the catalyst. The first aspect of the present invention forms the NOx retention layer and catalyst layer independently in a layer form and accelerates an NOx occlusion reaction by ozone supply means without resort to the catalyst layer. This makes it possible to occlude and reduce NOx while inhibiting the NOx retention substance from acting as a catalyst poison to keep the catalyst's exhaust gas purification function intact.

The second aspect of the present invention makes it possible to suppress the influence of the catalyst poison with higher effectiveness than in the first aspect.

According to the third aspect of the present invention, NO in the exhaust gas can be oxidized to generate NO₃, N₂O₅, and other nitrogen oxides of higher order than NO₂ (generate HNO₃ as well if water exists). This makes it possible to increase the amounts of NO₃, N₂O₅, and other nitrogen oxides of higher order than NO₂, which are contained in the exhaust gas that flows into an NOx retention member. As a result, an NOx occlusion reaction can be accelerated to increase the exhaust gas purification capacity.

According to the fourth aspect of the present invention, a sufficient amount of ozone can be supplied as needed to generate NO₃, N₂O₅, and other nitrogen oxides of higher order than NO₂ (generate HNO₃ as well if water exists) by oxidizing NO. As a result, the NOx occlusion reaction can be effectively accelerated to increase the exhaust gas purification capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating configuration of an exhaust emission control apparatus according to a first embodiment of the present invention.

FIGS. 2A and 2B are diagrams illustrating configuration of the apparatus according to the first embodiment.

FIG. 3 is a flowchart illustrating a routine that the ECU executes in the first embodiment.

FIG. 4 is a diagram to describe result of experiment for the first embodiment.

FIG. 5 is a diagram to describe result of experiment for the first embodiment.

FIGS. 6A and 6B are diagrams to describe result of experiment for the first embodiment.

FIGS. 7A to 7C are are diagrams to describe result of experiment for the first embodiment.

FIG. 8 is a diagram to describe result of experiment for the first embodiment.

DESCRIPTION OF NOTATIONS

-   10 an internal combustion engine -   12 an exhaust path -   20 a catalytic device -   22 an NOx occlusion reduction type catalyst -   24 cell -   25 a base material -   26 an NOx retention layer -   27 a catalyst layer -   28 a flow path -   30 an ozone supply device -   32 an ozone injection orifice -   34 an air inlet -   50 ECU

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment [Configuration of First Embodiment]

FIG. 1 is a diagram illustrating an exhaust emission control apparatus according to a first embodiment of the present invention. As shown in FIG. 1, the exhaust emission control apparatus according to the first embodiment includes a catalytic device 20, which is placed in an exhaust path 12 of an internal combustion engine 10. An NOx occlusion reduction type catalyst 22 is placed in the catalytic device 20. As far as the exhaust emission control apparatus is configured as described above, an exhaust gas passing through the exhaust path 12 flows into the catalytic device 20 and then into the NOx occlusion reduction type catalyst 22.

The NOx occlusion reduction type catalyst 22 is formed when a base material (hereinafter also referred to as a support) is coated with a layer containing an NOx retention substance (hereinafter also referred to as the “NOx retention layer”) and a layer containing a noble metal (hereinafter also referred to as the “catalyst layer”). The NOx retention layer is capable of occluding NOx in the exhaust gas and releasing the occluded NOx in a high-temperature or rich atmosphere. The catalyst layer is capable of inducing a reaction between NOx and HC or CO and decomposing the NOx into N₂, H₂O, CO₂, etc. Employing the configuration described above makes it possible to effectively purify NOx contained in the exhaust gas.

The configuration of the NOx occlusion reduction type catalyst 22 will be further described with reference to FIGS. 2(A) and 2(B). FIG. 2(A) is a cross-sectional left side view that is taken along line A-A in FIG. 1 to illustrate the NOx occlusion reduction type catalyst 22. As shown in FIG. 2(A), the NOx occlusion reduction type catalyst 22 includes a ceramic base material 25 having a circular outline. As indicated in FIG. 2(A), the base material 25 is honeycombed to include a plurality of rectangular cells 24. The cells 24 run through the base material 25 in the direction of the depth of FIG. 2(A) so that the exhaust gas can be distributed in that direction.

FIG. 2(B) is an enlarged view of one cell 24. Each cell 24 is one of a plurality of partitions provided by the base material 25. Each cell 24 includes an NOx retention layer 26 and a catalyst layer 27. These layers are provided on the surface of the base material 25 individually and sequentially from the side of the base material 25. A flow path 28 is formed in each cell 24 and positioned inside the catalyst layer 27. The flow path 28 is extended in the direction of the depth of FIG. 2(B) so that the exhaust gas is distributed through the flow path 28.

The NOx retention layer 26 is formed by coating the base material 25 with an NOx retention material containing BaCO₃. BaCO₃ functions as an NOx retention substance (also referred to as an NOx occlusion agent) that occludes NOx in the exhaust gas as nitrate (or more specifically, Ba(NO₃)₂). The occluded Ba(NO₃)₂ is actively released when mainly the exhaust gas is rich or when the NOx retention substance temperature is high.

The catalyst layer 27 is formed by coating the NOx retention layer 26 with a catalytic material containing Pt or other noble metal. Pt or other noble metal functions as an active site that simultaneously activates the oxidation reaction of CO and HC and the reduction reaction of NOx. Thus, the catalyst layer 27 functions as a three-way catalyst that simultaneously purifies NOx, CO, and HC. The catalyst layer 27 is gas permeable to let NOx move between the flow path 28 and NOx retention layer 26.

The apparatus according to the first embodiment also includes an ozone supply device 30 as shown in FIG. 1. The ozone supply device 30 is in communication with an air inlet 34. The ozone supply device 30 can acquire air from the air inlet 34, generate ozone (O₃), and supply the generated ozone downstream. The configuration, function, and other characteristics of an ozone generator which generates ozone from air will not be described in detail because a variety of related technologies are publicly known.

The ozone supply device 30 has an ozone injection orifice 32 which injects a gas within the catalytic device 20. The ozone injection orifice 32 is positioned upstream of the NOx occlusion reduction type catalyst 22 in the catalytic device 20. When this configuration is employed to inject ozone from the ozone injection orifice 32, the ozone or air can be added to the exhaust gas passing through the exhaust path 12. The added ozone or air then mixes with the exhaust gas so that the resulting gas mixture flows into the NOx occlusion reduction type catalyst 22.

The exhaust emission control apparatus according to the first embodiment includes an ECU (Electronic Control Unit) 50. The ECU 50 is connected to the ozone supply device 30. The ECU 50 transmits a control signal to the ozone supply device 30 for the purpose of controlling the timing and amount of ozone injection. The use of the above-described configuration makes it possible to supply ozone at desired timing.

To efficiently induce an NOx occlusion reaction, it is preferred that NOx in the exhaust gas be oxidized to an increased extent. In the first embodiment, the ozone supply device 30 can add ozone to the exhaust gas as needed. This makes it possible to effectively purify the exhaust gas by oxidizing NOx in the exhaust gas during a gas phase reaction.

The ECU 50 is also connected, for instance, to various sensors which are provided for the internal combustion engine 10. Therefore, the ECU 50 can acquire information, for instance, about the temperature, engine speed Ne, air-fuel ratio A/F, load, and intake air amount of the internal combustion engine 10.

[Features of First Embodiment] (Features of Configuration)

As described above, the NOx retention substance (BaCO₃ in the first embodiment) contained in the NOx retention member is capable of occluding NOx in the exhaust gas. The noble metal contained in the catalyst (Pt, Rh, Pd, etc. in the first embodiment) functions as an active site during exhaust gas purification. To achieve NOx occlusion and reduction and exhaust gas purification with high efficiency, it is important that the above functions be exercised effectively in a coordinated manner.

Various conventional catalysts formed by integrating the abovementioned NOx retention member and catalyst are known. These catalysts are disclosed, for instance, in Japanese Patent No. 3551346 and also referred to as a “NOx occlusion reduction type catalyst” or “NSR catalyst.” The NOx occlusion reduction type catalyst can accelerate the NOx occlusion reaction by promoting the oxidation of NOx by using the catalyst. Further, when NOx is to be released, the catalyst can purify the exhaust gas.

However, when the NOx retention member is combined with the catalyst as described above, the exhaust gas purification capacity of the catalyst (the capacity for purifying NOx, HC, and CO) becomes smaller than that of a conventional three-way catalyst that does not contain an NOx retention substance. The reason would be that the NOx retention substance acts as a catalyst poison for the catalyst (noble metal element) and impairs the catalyst's activation function. To achieve exhaust gas purification with high efficiency, it is preferred that such an adverse effect be avoided to fully exercise the catalyst's function.

In view of the above circumstances, the exhaust emission control apparatus according to the first embodiment configures the NOx occlusion reduction type catalyst 22 by providing the base material 25 with the NOx retention layer 26 and catalyst layer 27 separately so as to provide these two layers as independent layers. As mentioned earlier, the catalyst's exhaust gas purification capacity decreases when the NOx retention substance acts as a catalyst poison. The first embodiment prevents the NOx retention substance from acting as a catalyst poison for the catalyst layer 27 because the NOx retention layer 26 and catalyst layer 27 are formed independently of each other. The following describes operations that are performed for NOx occlusion and NOx release when the configuration according to the first embodiment is employed.

(Operation Performed for NOx Occlusion)

As described above, the NOx occlusion reduction type catalyst 22 according to the first embodiment has the catalyst layer 27. The catalyst layer 27 includes Pt or other noble metal and can simultaneously purify NOx, CO, and HC (this function may be hereinafter referred to as the “exhaust gas purification function”). However, to enable the catalyst to exercise its exhaust gas purification function, it is necessary that the catalyst be heated to an adequate activation temperature. Therefore, when the internal combustion engine 12 starts up, particularly at a cold temperature, it is difficult to purify NOx contained in the exhaust gas because the temperature of the NOx occlusion reduction type catalyst 22 is low.

In the above situation, therefore, the present embodiment causes the NOx retention layer 26 to occlude NOx. Further, to accelerate such an NOx occlusion, the present embodiment uses the ozone supply device 30 to supply ozone in such a manner that ozone mixes with the exhaust gas flowing into the NOx occlusion reduction type catalyst 22. When ozone is added to the exhaust gas in the above manner, NOx in the exhaust gas is oxidized to facilitate NOx occlusion.

The NOx oxidized by ozone reaches the NOx occlusion reduction type catalyst 22 and flows into the flow path 28 within each cell 24. When the NOx flows in the flow path 28, it passes through the catalyst layer 27, which is not yet heated to its activation temperature, and then reaches the NOx retention layer 26. Subsequently, an occlusion reaction occurs in the NOx retention layer 26 so that NOx is occluded as nitrate. As the above-described operation is performed, it is possible to occlude NOx in the exhaust gas even in a situation where the catalyst layer 27 of the NOx occlusion reduction type catalyst 22 has not reached its activation temperature at startup of the internal combustion engine 12.

(Operation Performed for NOx Release)

As the aforementioned NOx occlusion takes place after startup of the internal combustion engine 12, the temperature of the NOx occlusion reduction type catalyst 22 rises. Therefore, when an adequate period of time elapses after startup of the internal combustion engine 12, the temperature of the catalyst layer 27 in the NOx occlusion reduction type catalyst 22 reaches an activation temperature. Consequently, when the catalyst layer 27 reaches its activation temperature and is ready to fully exercise its exhaust gas purification function, the first embodiment shuts off the supply of ozone and exercises control to slightly enrich the fuel injection amount of the internal combustion engine 12.

When the supply of ozone is shut off, an acceleration of the NOx occlusion reaction is stopped. Further, when the temperature of the NOx occlusion reduction type catalyst 22 is high, the temperature of the NOx retention layer 26 is also high. As the temperature rises and the atmosphere becomes enriched, the NOx retention layer 26 actively releases the occluded NOx. Therefore, the NOx release reaction actively occurs due to the above-described control.

When NOx is released from the NOx retention layer 26, the released NOx reaches the catalyst layer 27. The NOx in the catalyst layer 27 is then reduced to N₂, H₂O, CO₂, etc. by HC and other reductants contained in the exhaust gas. As described earlier, the present embodiment is configured so that the NOx retention layer 26 and catalyst layer 27 are formed independently of each other. This configuration prevents the NOx retention substance from acting as a catalyst poison for the catalyst layer 27. Consequently, the present embodiment makes it possible to purify the exhaust gas effectively without blocking the exhaust gas purification function of the catalyst layer 27.

As described above, the present embodiment prevents the NOx retention substance from acting as a catalyst poison because the NOx retention layer 26 and catalyst layer 27 are formed independently of each other. This makes it possible to prevent the exhaust gas purification capability of the catalyst layer 27 from being hindered. Further, the present embodiment causes the ozone supply device 30 to supply ozone and accelerates the NOx occlusion reaction without resort to the catalyst. Therefore, NOx can be occluded and reduced while fully exercising the exhaust gas purification function of the catalyst.

In addition, when an NOx oxidation method based on ozone is used, NOx can be oxidized with increased certainty during a gas phase reaction without resort to the catalyst even when the temperature is low at the time, for instance, of internal combustion engine startup. Moreover, when water vapor exists, nitric acid arises and easily reacts with the NOx retention substance. This makes it possible to occlude NOx with high efficiency.

In the first embodiment, which is described above, the NOx occlusion reduction type catalyst 22 corresponds to the “NOx occlusion reduction type catalyst” according to the first aspect of the present invention; and the ozone supply device 30 corresponds to the “ozone supply means” according to the first aspect. Further, in the first embodiment, which is described above, the cell 24 corresponds to the “cell” according to the first aspect of the present invention; the NOx retention layer 26 corresponds to the “first layer” according to the first aspect; and the catalyst layer 27 corresponds to the “second layer” according to the first aspect.

[Details of Process Performed by First Embodiment]

A process performed by the exhaust emission control apparatus according to the first embodiment will now be described in detail with reference to FIG. 3. FIG. 3 is a flowchart illustrating a routine that the ECU 50 executes in the first embodiment. The routine is executed when the internal combustion engine 10 starts at a low temperature (e.g., at a cold start).

First, the routine shown in FIG. 3 performs step S100 to supply ozone. More specifically, the ECU 50 transmits a control signal to the ozone supply device 30 so that ozone is supplied at a predetermined flow rate. Ozone injection then occurs in accordance with the control signal. As a result, NO in the exhaust gas is oxidized to NO₃ so that an occlusion reaction occurs efficiently within the NOx retention layer 26.

Next, the routine performs step S110 to judge whether an O₃ supply shutoff condition is established. More specifically, step S110 is performed to judge whether a certain period of time, which is required for the catalyst layer 27 to reach its activation temperature and is predetermined, for instance, during an experiment, has elapsed. When the obtained judgment result does not indicate that the O₃ supply shutoff condition is established, the routine concludes that the catalyst layer 27 has not reached its activation temperature, and repeats steps S100 and beyond.

When the obtained judgment result indicates that the O₃ supply shutoff condition is established, the routine proceeds to step S130, shuts off the supply of O₃, and controls the operating status of the internal combustion engine 10 so that the air-fuel ratio changes from stoichiometric to slightly rich. As a result, the NOx occluded in the NOx retention layer 26 is released. The released NOx then reaches the catalyst layer 27, and becomes reduced and purified. Subsequently, the routine comes to an end.

When the above process is performed, it is possible to unfailingly prevent the NOx retention substance from acting as a catalyst poison and achieve NOx occlusion and reduction while fully exercising the exhaust gas purification function of the catalyst layer 27. Further, when an NOx oxidation method based on ozone is used, NOx can be surely oxidized without resort to the catalyst even when the temperature is low at the time, for instance, of internal combustion engine startup. This makes it possible to obtain excellent emission characteristics.

[Results of Experiment for First Embodiment]

Results of experiment for the first embodiment of the present invention will now be described with reference to FIGS. 4 to 7.

(Configuration of Measurement System)

FIG. 4 shows a measurement system that was used for the experiment. The measurement system includes a model gas generator 230 and a plurality of gas cylinders 232 in order to generate a model gas which represents the exhaust gas of an internal combustion engine. The model gas generator 230 can mix the gases in the gas cylinders 232 to create the following simulant gas:

Simulant Gas Composition

C₃H₆ 1000 ppm

CO 7000 ppm

NO 1500 ppm

O₂ 7000 ppm

CO₂ 10%

H₂O 3%

Remainder, N₂

The model gas generator 230 is in communication with an electric furnace in which a test piece 222 is placed. FIG. 5 is an enlarged view of the test piece 222 and its vicinity. As shown in FIG. 5, the test piece 222 is configured so that an embodiment sample 224 is housed in a quartz tube. The experiment involves the use of a comparative example for which the same experiment is to be conducted as with the embodiment sample 224 with a later-described comparative sample substituted for the embodiment sample 224.

The measurement system shown in FIG. 4 includes an oxygen cylinder 240. The downstream end of the oxygen cylinder 240 is in communication with flow rate control units 242, 244. The flow rate control unit 242 is in communication with the ozone generator 246. The ozone generator 246 receives oxygen which is supplied from the oxygen cylinder 240, and generates ozone. The ozone generator 246 communicates with the downstream end of the model gas generator 230 and the upstream end of the test piece 222 through an ozone analyzer 248 and a flow rate control unit 250.

Meanwhile, the downstream end of the flow rate control unit 244 directly communicates with the ozone analyzer. In a situation where the above-described configuration is employed, turning ON the ozone generator 246 supplies a gas mixture of O₃ and O₂ to the upstream end of the test piece 222 whereas turning OFF the ozone generator 246 supplies only O₂ to the upstream end of the test piece 222.

When the flow rate control units 242, 244 and the ozone generator 246 are used as appropriate, the measurement system shown in FIG. 4 makes it possible to create the following two types of gases, which differ in composition. Each of these gases is to be injected into the test piece 222 and will be hereinafter simply referred to as an “injection gas.”

Injection Gas Composition

(1) O₃, 30,000 ppm; remainder, O₂

(2) O₂ only

The flow rate control unit 250 can supply the injection gas at a desired flow rate.

Exhaust gas analyzers 260, 262 and an ozone analyzer 264 are positioned downstream of the test piece 222. These analyzers can measure gas components, the gas flowing out of the test piece 222.

The following measuring instruments were used during the experiment:

-   Ozone generator 246; Iwasaki Electric, OP100W -   Ozone analyzer 248 (upstream); Ebara Jitsugyo, EG600 -   Ozone analyzer 264 (downstream); Ebara Jitsugyo, EG2001B -   Exhaust gas analyzers 260, 262; Horiba, MEXA9100D (HC/CO/NOx     measurement); Horiba, VAI-510 (CO₂ measurement)

(Sample Preparation Method)

FIGS. 6(A) and 6(B) illustrate an embodiment sample and a comparative sample that were used during the experiment. FIG. 6(A) shows a cell of the embodiment sample 224 which is also shown in FIG. 5. FIG. 6(B) shows a cell of the comparative sample 324. The comparative sample 324 uses the same honeycombed base material as the embodiment sample 224, but is coated in a manner different from that for the embodiment sample 224.

The embodiment sample 224 shown in FIG. 6(A) was prepared by performing the procedure described below. First of all, γ-Al₂O₃ was dispersed in ion exchange water. An aqueous solution of barium acetate was then added. The resulting mixture was heated to remove water from it, dried at 120° C., and pulverized to powder. The powder was then burned for two hours at 500° C. The burnt powder was immersed in a solution containing ammonium hydrogen carbonate, and then dried at 250° C. to obtain barium that was supported on Al₂O₃ (hereinafter also referred to as the “barium-supported catalyst”). The support quantity of barium was 0.2 mole per 120 g of γ-Al₂O₃.

Next, γ-Al₂O₃ was dispersed in ion exchange water. An aqueous solution containing dinitro-diammine platinum was then added to support Pt. The resulting mixture was dried, pulverized, and burned for one hour at 450° C. to obtain platinum that was supported on Al₂O₃ (hereinafter also referred to as the “platinum-supported catalyst”). The support quantity of platinum was 4 g per 120 g of γ-Al₂O₃.

Next, a 30 mm diameter, 50 mm long, and 4 mil/400 cpsi cordierite honeycomb 125 was coated with the barium-supported catalyst and burned for one hour at 450° C. to obtain a Ba-supported catalyst layer 126. The coating amount was such that Al₂O₃ was coated at a rate of 60 g/L. Next, the honeycomb 125, which was coated as described above, was further coated with the platinum-supported catalyst and burned for one hour at 450° C. to obtain a Pt-supported catalyst layer 127. The coating amount was such that Al₂O₃ was coated at a rate of 60 g/L. The embodiment sample 224 having two layers of catalyst coating was obtained by performing the above process.

Consequently, the obtained embodiment sample 224 was such that the overall Pt support quantity was 2 g, and that the Ba support quantity was 0.1 mole/Al₂O₃ 120 g, and further that the coating amount was 120 g/L (Al₂O₃).

Meanwhile, the comparative sample 324, which is shown in FIG. 6(B), was prepared by performing the procedure described below. First of all, γ-Al₂O₃ was dispersed in ion exchange water. An aqueous solution of barium acetate was then added. The resulting mixture was heated to remove water from it, dried at 120° C., and pulverized to powder. The powder was then burned for two hours at 500° C. The burnt powder was immersed in a solution containing ammonium hydrogen carbonate, and then dried at 250° C. to obtain the barium-supported catalyst.

The obtained barium-supported catalyst was dispersed in ion exchange water. An aqueous solution containing dinitro-diammine platinum was then added to support Pt. The resulting mixture was dried, pulverized, and burned for one hour at 450° C. In this manner, a comparative coating catalyst was obtained. The obtained catalyst was such that the barium support quantity was 0.1 mole per 120 g of γ-Al₂O₃, and that the platinum support quantity was 2 g per 120 g of γ-Al₂O₃. Next, a 30 mm diameter, 50 mm long, 4 mil/400 cpsi cordierite honeycomb 325 was coated with the comparative coating catalyst, which was prepared as described above, and burned for one hour a 450° C. to obtain a PtBa catalyst layer 326. The coating amount was such that Al₂O₃ was coated at a rate of 120 g/L.

Consequently, the prepared comparative sample 324 was similar to the embodiment sample 224 in that the overall Pt support quantity was 2 g, and that the Ba support quantity was 0.1 mole/Al₂O₃ 120 g, and further that the coating amount was 120 g/L (Al₂O₃). Thus, the embodiment sample 224 and comparative sample 324 were configured so that they contained the same amounts of Pt and Ba.

(Description of Experiment)

In the measurement system described above, the aforementioned simulant gas and injection gas were combined and supplied to the test piece 222 under the following conditions. The electric furnace was controlled to raise the catalyst temperature at the following rate and determine, for instance, the concentration of NOx flowing downstream.

-   Temperature: 30° C. to 500° C. -   Temperature rise rate: 10° C./min (constant) -   Simulant gas flow rate: 30 L/min -   Injection gas flow rate: 6 L/min

The injection gas was supplied when the temperature was between 30° C. and 300° C. When the temperature was between 300° C. and 500° C., only the simulant gas was distributed without supplying the injection gas.

(Purification Efficiency Calculation Method)

FIGS. 7(A) to 7(C) are images illustrating how the exhaust gas purification efficiency was calculated in the experiment. FIG. 7(A) is an image illustrating the amount of a component of the supplied exhaust gas, which was determined by multiplying the simulant gas concentration by the test time. In accordance with the image, the amount of a component of the exhaust gas supplied within the measurement time was calculated in the experiment by multiplying the product of the concentration of the component in the simulant gas and a simulant gas flow rate by the test time.

FIG. 7(B) is an image illustrating the amount of a component of the exhaust gas flowing downstream, which was determined by multiplying the concentration of the gas flowing downstream of the test piece 222 by the test time. In accordance with the image, the amount of the component flowing downstream was calculated by multiplying the product of a component concentration, which was detected by an exhaust gas analyzer, and a gas flow rate by the test time.

The above calculated values were then used to determine the exhaust gas purification efficiency as shown in FIG. 7(C). More specifically, the amount of a component flowing downstream (FIG. 7(B)) was subtracted from the amount of gas supplied within the measurement time (FIG. 7(A)). The obtained value was then divided by the amount of gas supplied within the measurement time (FIG. 7(A)) to calculate the exhaust gas purification efficiency as a percentage.

(Results of Experiment)

FIG. 8 is a graph illustrating a first portion of the results of the experiment. The graph in FIG. 8 indicates that the use of the embodiment sample 224 exhibited higher purification efficiencies for NOx, HC, and CO than the use of the comparative sample 324.

The results of experiment, which have been described above, indicate that the first embodiment induces an NOx occlusion reaction while averting the influence of a catalyst poison. It means that the catalyst fully exercises its exhaust gas purification function to obtain excellent emission characteristics. Further, as mentioned above, the embodiment sample 224 contains the same amounts of barium and platinum as the comparative sample 324. In other words, the first embodiment makes it possible to use the NOx retention substance and noble metal with high efficiency.

[Modifications of First Embodiment] (First Modification)

The first embodiment coats the base material 25 with the NOx retention layer 26 that contains BaCO₃. However, the material for the NOx retention layer is not limited to the one described above. For instance, an alkali metal, such as Na, K, Cs, or Rb, an alkali earth metal, such as Ba, Ca, or Sr, or a rare earth element, such as Y, Ce, La, or Pr can be used as needed, as described in Japanese Patent No. 3551346.

Therefore, when the NOx retention substance occludes NOx as nitrate, the composition of the nitrate is not limited to Ba(NO₃)₂, which is mentioned in connection with the first embodiment. It should be noted that Ba has a large occlusion capacity (1 mole of Ba can occlude 3 moles of NO₃), exhibits higher thermal stability than the other materials, and is suitable as an NOx retention substance for use with an exhaust emission control apparatus.

The material for the catalyst layer 27 is not limited to Pt, Rh, Pb, or other materials described earlier. Various catalyst materials known as noble metal materials constituting an exhaust gas purification catalyst may be used with the present invention. Further, ceramic, alumina (Al₂O₃), and other appropriate materials may be used as a support material for a noble metal or NOx retention substance.

Similarly, the base material may also be made of various publicly known substances such as a ceramic or metal. In the first embodiment, the base material 25 having cells 24 partitioned in a reticular pattern is coated with the NOx retention layer 26 and catalyst layer 27. However, the present invention is not limited to the use of such a base material. Variously shaped, publicly known base materials may also be used with the present invention.

(Second Modification)

The first embodiment uses the ozone supply device 30 to add ozone to the exhaust gas. Preferably, however, such an ozone addition may be made in the manner described below. It is known that NOx in the exhaust gas oxidizes due to a gas phase reaction when ozone (O₃) is added to the exhaust gas. More specifically, the NOx reacts with the ozone to induce the following reactions:

NO+O₃→NO₂+O₂  [1]

NO₂+O₃→NO₃+O₂  [2]

NO₂+NO₃→N₂O₅  [3]

In the subsequent explanation, reaction formula [1] may be referred to as the “first formula;” reaction formula [2], the “second formula;” and reaction formula [3], the “third formula.” In the third formula, only the arrow indicating a rightward reaction is included; however, a leftward reaction may also occur.

NOx occlusion in the NOx retention substance is achieved when a high-order nitrogen oxide, which is generated when NOx is oxidized, or HNO₃, which is generated when such a nitrogen oxide reacts with water, is occluded by the NOx retention substance. When, for instance, NO₃ changes to Ba(NO₃)₂ or other nitrate, it is occluded by the NOx occlusion substance. To induce an NOx occlusion reaction with high efficiency, therefore, it is preferred that an increased amount of NOx in the exhaust gas change to NO₃, N₂O₅, and other nitrogen oxides of higher order than NO₂.

In view of the above circumstances, the second modification induces the reaction indicated by the second formula by adding ozone in such a manner that the mole ratio of ozone to NO in the gas mixture is greater than 1. More specifically, ozone addition is made so that the following relational expression is met by the ratio between mol(O₃), which is a mole equivalent of the amount of ozone in the gas mixture, and mol(NO), which is a mole equivalent of the amount of nitrogen monoxide in the gas mixture:

mol(O₃)/mol(NO)>1  [4]

In the subsequent explanation, formula [4] above may be referred to as the “fourth formula.”

When the mole ratio of ozone to NO in the gas mixture is not greater than 1 (mol(O₃)/mol(NO)≦1), NO₃ and N₂O₅ will not be generated due to the reactions indicated in the second and third formulae although NO₂ is generated due to the reaction indicated in the first formula. As such being the case, the second modification is configured so that the substance quantity of ozone to be added is greater than the substance quantity of NO in the exhaust gas. Therefore, an adequate amount of ozone can be supplied to generate NO₃ and N₂O by oxidizing NO (to induce the reactions indicated in the second and third formulae). As a result, the amounts of high-order nitrogen oxides in the exhaust gas can be certainly increased to achieve NOx occlusion effectively.

The process described above is implemented when the ECU 50 performs a “process for adjusting an ozone supply amount so that the mole ratio of ozone to nitrogen monoxide (NO) in a gas mixture flowing into the NOx occlusion reduction type catalyst is greater than 1” (ozone supply amount adjustment process). This process can be performed, for instance, before step S100 of the routine shown in FIG. 3. The ozone supply amount for providing the above mole ratio can be defined, for instance, by allowing the ECU 50 to estimate the molar quantity of NOx contained in the exhaust gas in accordance with the operating status (engine speed Ne, air-fuel ratio A/F, load, intake air amount, etc.) of the internal combustion engine 10 and calculate the flow rate of ozone to be supplied in accordance with the estimated molar quantity of NOx.

(Third Modification)

Alternatively, the ozone supply amount may be further increased so that the mole ratio of ozone to nitrogen monoxide in the gas mixture is not smaller than 2 (Mol(O₃)/Mol(NO)≧2). When the mole ratio of ozone (O₃) to nitrogen monoxide (NO) in the gas mixture is greater than 1, the ozone still remains in the gas mixture even after NO is oxidized to NO₂ as indicated in the first formula. Therefore, the reactions indicated in the second and third formulae occur to generate NO₃ and N₂O₅. However, when a trace amount of ozone remains after the reaction indicated in the first formula, the amounts of NO₃ and N₂O₅ to be generated during the reactions indicated in the second and third formulae are decreased.

In view of the above circumstances, the third modification adjusts the ozone supply amount so that the mole ratio between ozone and NO in the gas mixture is not smaller than 2 (Mol(O₃)/Mol(NO)≧2). This ensures that an adequate amount of ozone remains after the reaction indicated in the first formula and contributes to the reactions indicated in the second and third formulae, thereby certainly increasing the amounts of high-order nitrogen oxides. As described above, the third modification makes it possible to supply an adequate amount of ozone for generating NO₃ and N₂O₅ by oxidizing NO and effectively accelerate the NOx occlusion reaction.

The process described above is implemented when the ECU 50 performs a “process for adjusting an ozone supply amount so that the mole ratio of ozone (O₃) to nitrogen monoxide (NO) in the gas mixture flowing into the NOx occlusion reduction type catalyst is not smaller than 2.” This process can be performed, for instance, before step S100 of the routine shown in FIG. 3.

(Fourth Modification)

The first embodiment is configured so as to supply ozone with the ozone supply device 30 installed outside the catalytic device 20 and the ozone injection orifice 32 positioned inside the catalytic device 20. However, the present invention is not limited to the use of such a configuration. Ozone may be added to the exhaust gas by using various publicly known ozone generation devices methods. For example, a configuration for generating ozone directly by plasma discharge may be formed within the exhaust path 12 or catalytic device 20.

The NOx retention member may not only occlude NOx but also adsorb NOx. More specifically, the NOx occlusion reduction type catalyst 22 may not only occlude NOx but also adsorb NOx. Therefore, the “retention” operation performed by the NOx retention member means not only the “occlusion” of NOx but also the “adsorption” of NOx.

It is preferred that the amount of NOx retention substance contained in the catalyst layer 27 be substantially zero. However, the present invention is not limited to the use of such a catalyst layer. The present invention may alternatively be configured so that the catalyst layer 27 contains a smaller amount of NOx retention substance than the NOx retention layer 26. 

1: An exhaust emission control apparatus for an internal combustion engine, comprising: an NOx occlusion reduction type catalyst which is positioned in an exhaust path of the internal combustion engine; and ozone supply means which supplies ozone so that the ozone mixes with an exhaust gas flowing into the NOx occlusion reduction type catalyst; wherein the NOx occlusion reduction type catalyst includes a cell in which the exhaust gas flows, an inner surface of the cell being coated with a first layer and a second layer, the first layer and the second layer being provided in the order named from the inner surface of the cell, the first layer containing an NOx retention substance, the second layer including a noble metal and permitting the passage of NOx, the amount of the NOx retention substance in the second layer is smaller than that in the first layer. 2: The exhaust emission control apparatus according to claim 1, wherein the amount of the NOx retention substance contained in the second layer is substantially zero. 3: The exhaust emission control apparatus according to claim 1, further comprising: ozone supply amount adjustment means for adjusting an ozone supply amount so that the mole ratio of ozone to nitrogen monoxide (NO) in a gas mixture flowing into the NOx occlusion reduction type catalyst is greater than
 1. 4: The exhaust emission control apparatus according to claim 3, wherein the ozone supply amount adjustment means adjusts the ozone supply amount so that the mole ratio of ozone (O₃) to nitrogen monoxide (NO) in the gas mixture flowing into the NOx occlusion reduction type catalyst is not smaller than
 2. 5: An exhaust emission control apparatus for an internal combustion engine, comprising: an NOx occlusion reduction type catalyst which is positioned in an exhaust path of the internal combustion engine; and an ozone supply unit which supplies ozone so that the ozone mixes with an exhaust gas flowing into the NOx occlusion reduction type catalyst; wherein the NOx occlusion reduction type catalyst includes a cell in which the exhaust gas flows, an inner surface of the cell being coated with a first layer and a second layer, the first layer containing an NOx retention substance, the second layer including a noble metal and permitting the passage of NOx, the amount of the NOx retention substance in the second layer is smaller than that in the first layer, and the first layer and the second layer being provided in the order named from the inner surface of the cell. 